The process of manufacturing semiconductor wafers involves the pulling of a single crystal ingot from the melt, then cropping it at the seed and tang ends. The cylindrical crystal is then ground to achieve a specified diameter, and then wafers are sliced from the cropped and ground ingot. To remove any slicing damage, each wafer is ground and then etched to remove lapping damage. Each wafer is then mirror polished. Silicon wafers may be subject during processing to laser marking, backside damage and film deposition, front side epitaxial deposition and other intentional and inadvertent processes, depending upon customer specifications.
Referring first to FIGS. 1 and 2, prior art wafer measurement apparatus indicated generally at 10 will be described. Conventionally, caliper-type gauges are used to measure the thickness of semiconductor wafers, wherein a movable upper contact pin 12 and a fixed lower contact pin 14 are made to contact a wafer under test 16 (shown in phantom by dashed lines) on its upper and lower surfaces 16a, 16b, respectively. Wafer 16 is supported by a heavy aluminum base plate 18 onto which a metal plate 20 is affixed, as by screws 22 (not shown in FIG. 1, but shown in FIG. 2). Metal plate 20 typically contains ball bearings such as ball bearings 24 to produce virtually friction-free movement thereon of wafer under test 16.
Typically, a pair of pin slots 26, 28 are provided for insertion of a pair of diameter pins 30, 32 in one of plural pairs of pin holes 34, 36; 38, 40; 42, 44; 46, 48; 50, 52. It will be understood that the pair of diameter pins is placed to accommodate any one of a number of nominal wafer diameters, e.g. 100 millimeter (mm), 125 mm, 150 mm (as depicted in FIGS. 1 and 2) and 200 mm, with the inserted diameter pins effectively centering the wafer under test relative to the upper and lower contact pins.
Wafer thickness is measured after first `zeroing` or `zeroing in` a precision analogue or digital gauge or meter (not shown). Without a wafer under test resting on the ball bearings of metal plate 20, the gauge should indicate precisely zero thickness, since upper and lower contact pins 12, 14 will contact one another under the urging of a relatively strong tension probe spring 54 via a washer 56 to which upper contact pin 12 mounted on the lower tip end of an upper probe head 58 is connected. Any adjustments that are needed to zero the connected meter are made, and then upper contact pin 12 is raised by the user via a lift block 60. Lift block 60 is connected via a retractor extension 62 to a pivotal probe retractor 64, an arm of which engages washer 56 as illustrated. Probe retractor 64 is connected by a pin 66 for pivotal rotation in a generally vertical plane to a retractor joint block 68 mounted on an L-shaped extension block 70. A relatively weak retraction spring 72 connected between retractor 64 and a spring holder 74 mounted on extension block 70 assists the operator in raising lift block 60.
Those of skill in the particular art, especially those familiar with the illustrated Tokyo Seimitsu "Miniax" model DH151 wafer thickness test equipment, will appreciate finally that thickness tester 10 typically further includes a sensor pin 76, a spring fixture 78, a thread bolt 80 and corresponding nut 82, a mechanical, electromagnetic, piezoelectric or other suitable displacement sensor 84, a structural probe column 86 for mounting certain test components of the head of the equipment, and a cable 88, shown only fragmentarily as extending toward an RS-232- or other-type meter, not shown. Such features are typical of wafer thickness test equipment, and, indeed, of thickness test equipment in general.
Next, wafer under test 16 is made to rest, centered between an inserted pair of diameter pins 30, 32, on the ball bearings of metal plate 20 such that its lower surface 16b is in contact with lower contact pin 14. Probe spring 54 urges upper contact pin 12 against upper surface 16a of wafer 16, and the thickness of the wafer is indicated by the meter as the difference between the normally at-rest, in-contact positions of the contact pins and the wafer-produced, separated distance therebetween, wherein the separation is attributable to the thickness of the wafer. It is noted that the tips of upper and lower contact pins 12, 14 preferably are spherical to achieve point contact with wafer 16, thereby to maximize accuracy in the thickness measurement, and to reduce contact pin-induced defects, such as scratches, on the surface of the wafers.
Periodic calibration of the thickness gauge is necessary, and typically uses a certified gauge block 90 representative of national standards. Gauge blocks come in a variety of thicknesses, e.g. 400 micrometers (.mu.m) to 1000 .mu.m in 50 .mu.m increments. To calibrate thickness tester 10, gauge block 90 of standard thickness is placed into thickness tester 10, and the thickness of gauge block 90 is measured, with the gauge's thickness reading being adjusted to the standard by adjustment of the reading of the meter that forms a part of thickness tester 10 until it reads precisely the thickness of the standard represented by gauge block 90.
Typically, such gauge blocks are momentarily inserted between the upper and lower contact pins of the thickness tester 10 by hand. The problem with this prior art approach is that calibration error may be introduced during the calibration procedure if gauge block 90 is not precisely perpendicular to the contact pins during measurement and calibration adjustment. In other words, it is possible to insert the gauge block in a slightly out-of-perpendicular orientation between the upper and lower contact pins, which introduces error. Moreover, it is possible--since gauge block 90 typically is simply held by hand--for the gauge block to wobble between the upper and lower contact pins, thus producing a dubious reading even if held precisely in a perpendicular orientation immediately before and/or after the reading of the meter is made.
Such an out-of-perpendicular orientation of a gauge block is shown in FIG. 3, an enlarged, fragmentary side elevation corresponding to FIG. 1 and bearing identical reference designators (and with base plate 18 omitted for the sake of clarity). Gauge block 90 may be seen to be tipped, or inclined, such that it is not oriented parallel to metal plate 20. It may be appreciated that, due to this slight mis-orientation--which is typical because gauge block 90 typically is inserted between upper and lower contacts 12, 14 by hand--the distance between upper and lower contacts 12, 14 does not accurately represent the thickness of gauge block 90. As a result, the operator of thickness tester 10 might erroneously adjust the meter connected thereto to the standard gauge or thickness represented by the gauge block. Thereafter, unless and until thickness tester 10 is properly calibrated, it will tend to yield erroneously high wafer thickness readings. Such resulting thickness calibration errors are by no means negligible, as will be seen.
Referring to FIG. 4, one can calculate the possible measurement error when gauge blocks are positioned between the probes at an angle not equal to 90.degree. or perpendicular to the probe axis. The probe diameter is known and the angle, .alpha., can be assumed for any error calculation. With the radius, R, known, and the angle, .alpha., given, the following expression holds true: ##EQU1## Since two contact pins, i.e., the upper and lower probe, are involved as shown in FIG. 4A, the total thickness error is: EQU E=2X (3).
For example, if the probe tip radius is 1000 .mu.m and the gauge block angle, .alpha., is 85.degree., then the resulting error would be approximately 7.64 .mu.m, which represents approximately a 10% thickness measurement error, in the case of a typical 150 mm diameter wafer.
Another source of potential error .DELTA.E.sub.T is schematically shown in FIG. 4B. If the pins are not exactly opposite to each other during reset (zero in), then all subsequent readings will show an erroneous result, possibly cumulative with an inaccurate angle of the gauge block illustrated in FIG. 4A. Such equipment set-up limitations, shortcomings and intrinsic measurement errors all are overcome by the invented gauge block holder apparatus.